Intravascular balloon catheters that include ultrasonic imaging devices provide an effective alternative to coronary bypass surgery. Transluminal angioplasty surgery utilizes an elongated, flexible catheter having an inflatable balloon at its distal end that is inserted at an appropriate position in a vascular system of a patient. After the catheter is inserted into the vascular system, its balloon is routed to a stenosis. Once the balloon is properly positioned relative to the stenotic lesion, it is inflated with fluid under relatively high pressure. As the balloon expands, it dilates the stenosis, thus allowing blood to flow more freely.
Conventional methods of locating the exact position of a stenosis typically consist of injecting a contrast compound, such as a substance that is opaque to radiation, into the patient's bloodstream so that fluoroscopy can be used to enable the physician to view the flow of blood. To treat the stenosis, the physician first inserts a guide wire into the occluded vessel. After the guide wire has reached the stenosis or the occluded vessel, a tubular catheter having a lumen for the guide wire is pushed along the guide wire and guided by the guide wire until fluoroscopy reveals that the balloon is located within the stenosis. The balloon is then inflated to dilate the stenosis. When the balloon is deflated and removed from the vessel, blood is able to flow more freely through the enlarged stenosis.
While fluoroscopy enables the physician to locate the position of the stenosis, it fails to provide the physician with detailed, real-time images of the anatomy causing the stenosis during and after the dilation procedure. The limitations of fluoroscopy has lead to attempts to improve upon imaging of the stenosis being treated. One such method uses two catheters, wherein one catheter has an ultrasonic imaging device at its distal end and the other catheter has the inflatable balloon at its distal end. In practicing this method, the balloon catheter is first inserted into the vessel along the guide wire, positioned within the stenosis, and the balloon is inflated to dilate the stenosis. The balloon catheter is then removed and the imaging catheter is inserted to enable the physician to examine the stenosis and determine if further treatment is needed. Obviously, it would be more desirable to have only one catheter that has both dilating and imaging capabilities, thereby eliminating the catheter exchange procedure and further providing realtime images of the dilating procedure.
One example of a catheter having both dilating and imaging capabilities is disclosed in U.S. Pat. No. 4,841,977 to Griffith et al. which positions the imaging device inside the inflatable balloon. The positioning of the imaging device inside the balloon instead of behind the balloon appears to be necessary to prevent the imaging device from interfering with the flow of pressurized fluid into the balloon. While this design provides a catheter having both dilating and imaging capabilities, the low profile of balloon when deflated is limited to a diameter of the imaging device, thus preventing the deflated balloon from entering very narrow stenoses and limiting the accessibility of the catheter to larger diameter stenoses.
U.S. Pat. No. 4,917,097 to Proudian et al. also discloses a catheter having both dilating and imaging capabilities, wherein the imaging device is positioned at a distal end of the balloon. Similarly to Griffith et al., the capability of the Proudian balloon to enter very small diameter stenoses is limited by the relatively large diameter of the imaging device. In order to optimize these types of catheters, it is important for the balloon to be able to enter even the very narrow stenoses.
Another limitation of balloon catheters including imaging devices is the highly directional characteristic of imaging devices. Imaging devices typically include an imaging array that enables the device only to image an integral (or "slice") of the stenosis immediately adjacent to the periphery of the array. Therefore, the entire balloon catheter must be moved in order for the physician to examine additional integrals of the stenosis.